The present invention relates to a method for making a silicon carbide (SiC) substrate and, more particularly, to a method for making a substrate including a single crystal silicon carbide (SiC) layer formed through a liquid-phase epitaxial growth method.
Silicon Carbide (SiC) takes various kinds of crystal structures (referred to as Polytype), and has a band gap between 2.4 eV and 3.3 eV depending on the crystal strucure. The silicon carbide is thermally, chemically and physically stable, and resistant to high-energy particle radiation. Moreover, stable P-type and N-type substrates can be formed through the use of silicon carbide even though the silicon carbide has a wide band gap.
Accordingly, silicon carbide (SiC) is favorable for the semiconductor material of, for example, a high temperature operating device, a high power device, a high reliability semiconductor device and a radiation resistant device. By effectively utilizing the wide band gap, the silicon carbide (SiC) may provide a novel opto-electric converter sensitive to rays between the visible short-wave length light region and the near ultraviolet ray region. In addition, the silicon carbide (SiC) is free from certain problems related to pollution and resources.
Even though silicon carbide has various advantages as discussed above, a silicon carbide substrate has never been produced for commercial use.
Three methods are well known in the art to fabricate a silicon carbide substrate on a laboratory scale.
(1) LELY method PA0 (2) Solution Growth method PA0 (3) ACHESON method PA0 (1) The 3C--SiC single crystal thin-film is formed on the silicon substrate through the use of the chemical vapor deposition (CVD) technique while the silicon substrate is held at 1200.degree. C. through 1400.degree. C. PA0 (2) The 3C--SiC single crystal thin-film is formed on the silicon substrate by converting the silicon substrate surface into SiC, wherein carbon atoms, which are derived from graphite or hydro-carbon due to the thermal decomposition, are diffused into the silicon substrate surface at 1200.degree. C. through 1400.degree. C. PA0 (3) Silicon vapor is forced to pass through argon and hydro-carbon gas activated by the D.C. or A.C. glow discharge, whereby the SiC single crystal thinfilm is deposited on the silicon substrate.
Silicon carbide powder is vaporized in a graphite crucible at 2200.degree. C. through 2600.degree. C., and silicon carbide single crystals are recrystallized on the crucible wall.
Silicon carbide substrates are obtained from molten silicon, or a mixture comprising silicon and impurity such as iron, cobalt or platinum, in a graphite crucible.
A silicon carbide substrate is incidentally formed by the Acheson method which is widely employed for providing a lapping powder.
The Lely method and the solution growth method are not suited for providing a large size silicon carbide substrate because many crystallization nuclei are formed at the beginning of the crystal growth operation. Moreover, the two methods are not preferable for providing a large size silicon carbide substrate of the single crystal structure because various kinds of polytypes are inevitably formed. The silicon carbide substrate incidentally obtained through the Acheson method does not show high purity, and the crystal quality thereof is not good enough for use as a semiconductor material.
Recently, a new method has been proposed, wherein a single crystal thin-film of 3C--SiC (shows the polytype belonging to the cubic structure and has the energy gap around 2.2 eV) is formed on a single crystal silicon foreign substrate through the use of the heteroepitaxy technique. SiC includes Si and C by a 1:1 ratio. However, the alignment is not fixed. Therefore, the term "polytype" is used. The expression "3C" means one type of polytype. The SiC formed by the present method shows the 3C type. Three methods are proposed for the heteroepitaxial growth of the single crystal thin-film SiC on the silicon substrate.
silicon source: SiH.sub.4, SiCl.sub.4, (CH.sub.3).sub.3 SiCl, (CH.sub.3).sub.2 SiCl.sub.2 PA1 carbon source: CCl.sub.4, hydro-carbon gas (C.sub.2 H.sub.2, C.sub.2 H.sub.6, CH.sub.4, C.sub.3 H.sub.8, etc.) PA1 carrier gas: hydrogen, argon
The 3C--SiC single crystal thin-film formed on the silicon foreign substrate through the use of the abovediscussed heteroepitaxy techniques has a thickness of only 1 through 10 .mu.m and the crystal quality thereof is not completely preferred. This is because a lot of misfit dislocations are created near the boundary of the silicon substrate and the epitaxial 3C--SiC layer due to a great difference of the latice constant between the silicon crystal and the 3C--SiC crystal, and the misfit dislocation influences the entire epitaxial growth. This is further because a strain is accumulated in the SiC epitaxial layer during the cooling process due to a difference of the thermal expansion coefficient between the silicon substrate and the SiC crystal.
Moreover, the above-mentioned methods are not suited for providing a wider energy gap .alpha.-SiC, for example, 6H--SiC (having the energy gap of about 3.02 eV), 4H--SiC (having the energy gap of about 3.26 eV), 8H--SiC (having the energy gap of about 2.8 eV), since the silicon substrate cannot be employed as the substrate. More specifically, the epitaxial growth for forming the wider energy gap SiC must be controlled above 1600.degree. C. though the silicon substrate has a melting point of 1410.degree. C.